The surface of a piece of wood contains innumerable small openings which are the cells of the wood. On open-grain woods, such as chestnut and walnut, the openings are very pronounced. If varnish were to be applied directly to the surface of the wood, a portion of the varnish would flow into these small openings and fill them. The result would be that the varnish, when dry, would have an uneven and pitted surface. To avoid this condition, use a wood filler, after the stain is dry, to fill up the cells so that the varnish can be applied to an even surface. It is possible to apply varnish directly to the wood and sand out any irregularities after it is dry. This entails considerable sanding, as it may be necessary to sand not only the first coat of varnish but the second as well, before obtaining a really smooth surface. Using a filler, however, produces good results with less work.
Wood fillers are obtainable in paste or liquid form. Use the paste filler on all open-grain woods and the liquid filler on close-grain woods.
Paste fillers can be had in several different colours to blend with the commonly used wood finishes. There are also natural fillers which are transparent, and these are used either with a natural finish or with some of the blonde finishes. It is sometimes impossible to get o. filler of the right colour for a particular surface, and a natural filler can then be tinted with stain to produce the desired effect. White fillers are used on many blonde finishes instead of a natural filler.
Fillers can produce the necessary amount of colour alone, but they are, in most cases, used over a stain.
Do not use a filler that is darker than the stained wood.
Before using, the paste must first be thinned with turpentine or petrol until it is of a thick brushing consistency. Do not make up more filler than can be used within a few minutes, for it sets very quickly and cannot then be used.
Apply the filler with a brush, working with the grain and across it, so that the filler is forced down into the wood cells. Pay special attention to any portion of the woodwork that has been milled out, as these places are easily overlooked.
A filler requires about fifteen minutes to set, but the exact time will vary according to its consistency. A filler will become light in spots after a few minutes. This is the time to wipe off the excess.
Wipe the filler off the surface with a piece of burlap or some excelsior. Wipe across the grain and then make a few light strokes with the grain to finish off the job.
The purpose of wiping is to remove the excess filler on the surface of the wood, leaving only the portion that has penetrated to the pores. Do not wipe off the filler too soon or an insufficient amount will enter the pores, and do not leave it on too long or it will become hard, making removal extremely difficult. If a filler is too thick in the beginning it will not penetrate the wood, no matter how hard it is brushed.
Use a liquid filler on the close-grain woods and brush it on with the grain. When dry, sand the surface with No. 00 sandpaper.
Shellac can be, and often is, used as a filler for close-grain woods. After the shellac is dry, sand the surface until only the shellac absorbed by the wood remains.